Distribution System Harmonics: Controls for Large Power Converters

Because of the anticipated applicaton of large ac/dc power converters on utility distribution systems, the Electric Power Research Institute (EPRI) funded Project RP1024-1 to study distribution system surge and harmonic characteristics. The harmonics produced by large converters depends on converter type (line-or self-commutated), converter pulse number, thyristor firing angle controls, commutation angles, voltage regulation techniques, voltage unbalance, and-very importantly-network impedance frequency response. Of particular significance is the fact that a great deal of effort is still required to determine tolerable levels of harmonic voltages and currents on distribution systems. Harmonic controls for power converters-important because of their effect on converter cost and efficiency-are now specified on the basis of engineering data which need to be enhanced by further testing and analysis. In the face of observable and ever-increasing background levels of distribution system harmonics, more precise anticipation of future concerns is required. Test methods and instrumentation need to be standardized to ensure conformance to yet-to- be-determined permissible harmonic limits. This paper discusses line-and self-commutated power converters and techniques for controlling the harmonics associated with them and presents the principal results of EPRI Project RP1024-1.